In electrically driven vehicles, in particular, it is known to dispose the traction motor directly in the wheel to be driven. The rotor of the electric traction motor may be connected directly to the rim of the wheel. Such a concept is known from DE102006040223A1 and is referred to as direct drive. An alternative design is described in EP1382476B1, where the electric motor acts via a transmission on the wheel to be driven.
In either case, the drive must comply with electromagnetic compatibility (EMC) regulations. However, the electromagnetic fields generated by the stator may induce voltages in the rotor, which may cause a current to flow from the rotor via the rotor shaft toward ground via the rolling element bearings. In particular at high rotational speeds, the rolling element bearings disposed between the bearing rings lose mechanical contact with the bearing rings, so that the rolling elements are spaced by a lubricant film from at least one of the bearing rings. However, the electrically insulating lubricant is of limited dielectric strength. If the induced electric voltage results in partial discharges in the lubricant or causes a dielectric breakdown, then radiation is emitted which may not be acceptable under EMC aspects. Moreover, such a breakdown may result in contact erosion on bearing rings and rolling elements, resulting in premature aging of said components.
German Patent Application DE102010022320A1 describes a wheel hub motor for driving a motor vehicle having the features set forth in the preamble of claim 1. Here, bearing currents and the emission of electromagnetic waves are reduced by providing an electrically conductive connection between the rotor and the stator of the electric machine via a contact element.